Fan outlet guide vanes (OGVs) within high bypass turbofan engines are multifunctional components, which de-swirl the flow, provide noise suppression, and act as one of the main core mounting structures. For a fixed pitch turbofan engine the bypass flow cold nozzle is usually choked at high power and cruise operation conditions. This ensures the resulting swirl angle from the vanes remains within manageable limits throughout the flight envelope.
On a fixed pitch engine, the OGVs are profiled to accommodate a small range of swirl angles exiting the fan, and to efficiently redirect the flow such that when the air exists the engine its direction is parallel to the engine axis.
However, the introduction of a variable pitch fan generates a greater range of swirl angles exiting the fan, due to the fan operating under a range of pitch angles. Moreover, a variable pitch fan also allows a reverse flow regime to be established in the bypass flow for the purposes of generating reverse thrust. In reverse flow, static OGVs which are designed to work most efficiently in cruise can induce significant swirl into the flow, increasing core inlet distortion.
Accordingly, variable OGVs have been proposed. For example, a mechanism can be provided to rotate the outlet guide vanes around the radial direction. However, such a mechanism can detract from ability of the OGVs to perform as structural mounting components.